In the US, about 67% of the generated electricity is produced by burning fossil fuels, of which coal contributes about 39% (U.S. Energy Information Administration, May 2015 Monthly Energy Review, Washington, D.C., 2015 102). The corresponding numbers for China are 69% and 63% (U.S. Energy Information Administration, China International energy data and Analysis, Washington, D.C., 2015). Burning of coal not only emits more carbon dioxide per kWh electricity produced compared with natural gas (U.S. Energy Information Administration, http://www.eia.gov/tools/faqs/faq.cfm?id=74&t=112015), but also particulates and other toxic pollutants such as mercury and arsenic (U.S. Environmental Protection Agency, http://www.epa.gov/cleanenergy/energyand-you/affect/air-emissions.htm12014). Since it is unlikely for the US, China and other countries to stop burning fossil fuels to produce electricity in the foreseeable future, the most reasonable solution to mitigate CO2 and pollutant emission is to make power plants more efficient. US coal-fired power plants operate at an average efficiency of 32%, emitting about 1000 gm CO2 per kWh electricity produced. According to the World Coal Association, if one can raise the efficiency to 50%, the CO2 emission will be reduced to about 700 gm per kWh (World Coal Association, http://www.worldcoal.org/extract/cleaner-coal-technologiesvital-to-reducing-global-co2-emissions-5096/2015).
One can increase the thermal efficiency of these power plants by operating the steam generator at higher temperatures (and pressures). Most steam turbines in the US operate at 540° C. or below, and the proposed target by the US Department of Energy is to increase the operating temperature to 760° C. Extensive research studies are being conducted on the use of Ni-based superalloys for such applications, and results are quite promising (A. F. Stam, Proceedings from the Seventh International Conference on Advances in Materials Technology for Fossil Power Plants 2013, p. 74). However, a drawback of these superalloys is their cost, about $30-40/kg in 2015.